The invention relates to the field of terrain or ground proximity warning systems for aircraft and more particularly to terrain warning systems that generate warning signals as a function of the aircraft's rate of descent as compared to its altitude above the ground.
This invention relates in particular to the prior art ground proximity warning system disclosed in Astengo U.S. Pat. No. 3,715,718 and the application of Bateman Ser. No. 480,727, filed on June 19, 1974, entitled "Ground Proximity Warning Instrument," both of which are assigned to the assignee of this application. The terrain closure warnings described in Astengo and Bateman use a signal derived from a radar altimeter representing the rate of change in the aircraft altitude above ground where a symmetric rate limiter is utilized to limit the amplitude of this rate signal to the same maximum value for both increasing and decreasing rates of change in the aircraft's altitude above ground. The limited rate signal is then utilized as an input to a complementary filter which serves to remove high frequency components from the radar altitude rate signal.
However, it has been determined that, when an aircraft is flying at a relatively high air speed over undulating terrain having relatively steep slopes, a negative slope, i.e. terrain that is sloping downwardly away from the aircraft, will cause the low pass filter to take an appreciable charge having a non-warn polarity. This will have the effect of delaying the warning signal when the aircraft begins to traverse a positive or upwardly sloping terrain due to negative conditioning of the system. At the same time it is extremely important that nuisance warnings be reduced to an absolute minimum since every nuisance warning tends to reduce the aircrew's confidence in the system and hence the value of the system as a safety device. Thus the problem becomes how to increase the sensitivity of the system to avoid the delaying effects of downward slopes while concurrently reducing nuisance warnings to a minimum.
By the same token, it is considered worthwhile to extend the altitude at which the system will respond and at the same time reduce nuisance warnings resulting from rough terrain at these higher altitudes.